the same happened to me but all my games stoped working. you might have a hidden corrupted file somewere. i had a repair guy come over to look at it and he backed up and restored the data. you will not lose any progress on any game if you do so.

what i do is deleting all addons in your main left for dead file in you pc then restart steam, it fixes everyones problem, because one addon can conflict with another and it either causes a big crash or really bad and unplayable lag